Table of Contents

For the impatient

Simply add

<Cluster className="org.apache.catalina.ha.tcp.SimpleTcpCluster"/>

to your <Engine> or your <Host> element to enable clustering.

Using the above configuration will enable all-to-all session replication
using the DeltaManager to replicate session deltas. By all-to-all we mean that the session gets replicated to all the other
nodes in the cluster. This works great for smaller cluster but we don't recommend it for larger clusters(a lot of Tomcat nodes).
Also when using the delta manager it will replicate to all nodes, even nodes that don't have the application deployed.
To get around this problem, you'll want to use the BackupManager. This manager only replicates the session data to one backup
node, and only to nodes that have the application deployed. Downside of the BackupManager: not quite as battle tested as the delta manager.

Here are some of the important default values:

Multicast address is 228.0.0.4

Multicast port is 45564 (the port and the address together determine cluster membership.

The IP broadcasted is java.net.InetAddress.getLocalHost().getHostAddress() (make sure you don't broadcast 127.0.0.1, this is a common error)

The TCP port listening for replication messages is the first available server socket in range 4000-4100

Listener is configured ClusterSessionListener

Two interceptors are configured TcpFailureDetector and MessageDispatch15Interceptor

Cluster Basics

To run session replication in your Tomcat 8 container, the following steps
should be completed:

All your session attributes must implement java.io.Serializable

Uncomment the Cluster element in server.xml

If you have defined custom cluster valves, make sure you have the ReplicationValve defined as well under the Cluster element in server.xml

If your Tomcat instances are running on the same machine, make sure the Receiver.port
attribute is unique for each instance, in most cases Tomcat is smart enough to resolve this on it's own by autodetecting available ports in the range 4000-4100

Make sure your web.xml has the
<distributable/> element

If you are using mod_jk, make sure that jvmRoute attribute is set at your Engine <Engine name="Catalina" jvmRoute="node01" >
and that the jvmRoute attribute value matches your worker name in workers.properties

Make sure that all nodes have the same time and sync with NTP service!

Make sure that your loadbalancer is configured for sticky session mode.

Load balancing can be achieved through many techniques, as seen in the
Load Balancing chapter.

Note: Remember that your session state is tracked by a cookie, so your URL must look the same from the out
side otherwise, a new session will be created.

Note: Clustering support currently requires the JDK version 1.5 or later.

The Cluster module uses the Tomcat JULI logging framework, so you can configure logging
through the regular logging.properties file. To track messages, you can enable logging on the key: org.apache.catalina.tribes.MESSAGES

Overview

To enable session replication in Tomcat, three different paths can be followed to achieve the exact same thing:

Using session persistence, and saving the session to a shared file system (PersistenceManager + FileStore)

Using session persistence, and saving the session to a shared database (PersistenceManager + JDBCStore)

Using in-memory-replication, using the SimpleTcpCluster that ships with Tomcat (lib/catalina-tribes.jar + lib/catalina-ha.jar)

In this release of session replication, Tomcat can perform an all-to-all replication of session state using the DeltaManager or
perform backup replication to only one node using the BackupManager.
The all-to-all replication is an algorithm that is only efficient when the clusters are small. For larger clusters, to use
a primary-secondary session replication where the session will only be stored at one backup server simply setup the BackupManager.
Currently you can use the domain worker attribute (mod_jk > 1.2.8) to build cluster partitions
with the potential of having a more scalable cluster solution with the DeltaManager(you'll need to configure the domain interceptor for this).
In order to keep the network traffic down in an all-to-all environment, you can split your cluster
into smaller groups. This can be easily achieved by using different multicast addresses for the different groups.
A very simple setup would look like this:

What is important to mention here, is that session replication is only the beginning of clustering.
Another popular concept used to implement clusters is farming, i.e., you deploy your apps only to one
server, and the cluster will distribute the deployments across the entire cluster.
This is all capabilities that can go into with the FarmWarDeployer (s. cluster example at server.xml)

In the next section will go deeper into how session replication works and how to configure it.

Cluster Information

Membership is established using multicast heartbeats.
Hence, if you wish to subdivide your clusters, you can do this by
changing the multicast IP address or port in the <Membership> element.

The heartbeat contains the IP address of the Tomcat node and the TCP port that
Tomcat listens to for replication traffic. All data communication happens over TCP.

The ReplicationValve is used to find out when the request has been completed and initiate the
replication, if any. Data is only replicated if the session has changed (by calling setAttribute or removeAttribute
on the session).

One of the most important performance considerations is the synchronous versus asynchronous replication.
In a synchronous replication mode the request doesn't return until the replicated session has been
sent over the wire and reinstantiated on all the other cluster nodes.
Synchronous vs. asynchronous is configured using the channelSendOptions
flag and is an integer value. The default value for the SimpleTcpCluster/DeltaManager combo is
8, which is asynchronous. You can read more on the send flag(overview) or the
send flag(javadoc).
During async replication, the request is returned before the data has been replicated. async replication yields shorter
request times, and synchronous replication guarantees the session to be replicated before the request returns.

Bind session after crash to failover node

If you are using mod_jk and not using sticky sessions or for some reasons sticky session don't
work, or you are simply failing over, the session id will need to be modified as it previously contained
the worker id of the previous tomcat (as defined by jvmRoute in the Engine element).
To solve this, we will use the JvmRouteBinderValve.

The JvmRouteBinderValve rewrites the session id to ensure that the next request will remain sticky
(and not fall back to go to random nodes since the worker is no longer available) after a fail over.
The valve rewrites the JSESSIONID value in the cookie with the same name.
Not having this valve in place, will make it harder to ensure stickiness in case of a failure for the mod_jk module.

Remember, if you are adding your own valves in server.xml then the defaults are no longer valid,
make sure that you add in all the appropriate valves as defined by the default.

Hint:
With attribute sessionIdAttribute you can change the request attribute name that included the old session id.
Default attribute name is org.apache.catalina.ha.session.JvmRouteOrignalSessionID.

Trick:
You can enable this mod_jk turnover mode via JMX before you drop a node to all backup nodes!
Set enable true on all JvmRouteBinderValve backups, disable worker at mod_jk
and then drop node and restart it! Then enable mod_jk Worker and disable JvmRouteBinderValves again.
This use case means that only requested session are migrated.

The main element, inside this element all cluster details can be configured.
The channelSendOptions is the flag that is attached to each message sent by the
SimpleTcpCluster class or any objects that are invoking the SimpleTcpCluster.send method.
The description of the send flags is available at
our javadoc site
The DeltaManager sends information using the SimpleTcpCluster.send method, while the backup manager
sends it itself directly through the channel.
For more info, Please visit the reference documentation

This is a template for the manager configuration that will be used if no manager is defined in the <Context>
element. In Tomcat 5.x each webapp marked distributable had to use the same manager, this is no longer the case
since Tomcat you can define a manager class for each webapp, so that you can mix managers in your cluster.
Obviously the managers on one node's application has to correspond with the same manager on the same application on the other node.
If no manager has been specified for the webapp, and the webapp is marked <distributable/> Tomcat will take this manager configuration
and create a manager instance cloning this configuration.
For more info, Please visit the reference documentation

<Channel className="org.apache.catalina.tribes.group.GroupChannel">

The channel element is Tribes, the group communication framework
used inside Tomcat. This element encapsulates everything that has to do with communication and membership logic.
For more info, Please visit the reference documentation

Membership is done using multicasting. Please note that Tribes also supports static memberships using the
StaticMembershipInterceptor if you want to extend your membership to points beyond multicasting.
The address attribute is the multicast address used and the port is the multicast port. These two together
create the cluster separation. If you want a QA cluster and a production cluster, the easiest config is to
have the QA cluster be on a separate multicast address/port combination than the production cluster.
The membership component broadcasts TCP address/port of itself to the other nodes so that communication between
nodes can be done over TCP. Please note that the address being broadcasted is the one of the
Receiver.address attribute.
For more info, Please visit the reference documentation

In tribes the logic of sending and receiving data has been broken into two functional components. The Receiver, as the name suggests
is responsible for receiving messages. Since the Tribes stack is thread less, (a popular improvement now adopted by other frameworks as well),
there is a thread pool in this component that has a maxThreads and minThreads setting.
The address attribute is the host address that will be broadcasted by the membership component to the other nodes.
For more info, Please visit the reference documentation

The sender component, as the name indicates is responsible for sending messages to other nodes.
The sender has a shell component, the ReplicationTransmitter but the real stuff done is done in the
sub component, Transport.
Tribes support having a pool of senders, so that messages can be sent in parallel and if using the NIO sender,
you can send messages concurrently as well.
Concurrently means one message to multiple senders at the same time and Parallel means multiple messages to multiple senders
at the same time.
For more info, Please visit the reference documentation

Tribes uses a stack to send messages through. Each element in the stack is called an interceptor, and works much like the valves do
in the Tomcat servlet container.
Using interceptors, logic can be broken into more manageable pieces of code. The interceptors configured above are:
TcpFailureDetector - verifies crashed members through TCP, if multicast packets get dropped, this interceptor protects against false positives,
ie the node marked as crashed even though it still is alive and running.
MessageDispatch15Interceptor - dispatches messages to a thread (thread pool) to send message asynchronously.
ThroughputInterceptor - prints out simple stats on message traffic.
Please note that the order of interceptors is important. The way they are defined in server.xml is the way they are represented in the
channel stack. Think of it as a linked list, with the head being the first most interceptor and the tail the last.
For more info, Please visit the reference documentation

The cluster uses valves to track requests to web applications, we've mentioned the ReplicationValve and the JvmRouteBinderValve above.
The <Cluster> element itself is not part of the pipeline in Tomcat, instead the cluster adds the valve to its parent container.
If the <Cluster> elements is configured in the <Engine> element, the valves get added to the engine and so on.
For more info, Please visit the reference documentation

The default tomcat cluster supports farmed deployment, ie, the cluster can deploy and undeploy applications on the other nodes.
The state of this component is currently in flux but will be addressed soon. There was a change in the deployment algorithm
between Tomcat 5.0 and 5.5 and at that point, the logic of this component changed to where the deploy dir has to match the
webapps directory.
For more info, Please visit the reference documentation

Since the SimpleTcpCluster itself is a sender and receiver of the Channel object, components can register themselves as listeners to
the SimpleTcpCluster. The listener above ClusterSessionListener listens for DeltaManager replication messages
and applies the deltas to the manager that in turn applies it to the session.
For more info, Please visit the reference documentation

How it Works

To make it easy to understand how clustering works, We are gonna take you through a series of scenarios.
In the scenario we only plan to use two tomcat instances TomcatA and TomcatB.
We will cover the following sequence of events:

TomcatA starts up

TomcatB starts up (Wait that TomcatA start is complete)

TomcatA receives a request, a session S1 is created.

TomcatA crashes

TomcatB receives a request for session S1

TomcatA starts up

TomcatA receives a request, invalidate is called on the session (S1)

TomcatB receives a request, for a new session (S2)

TomcatA The session S2 expires due to inactivity.

Ok, now that we have a good sequence, we will take you through exactly what happens in the session replication code

TomcatA starts up

Tomcat starts up using the standard start up sequence. When the Host object is created, a cluster object is associated with it.
When the contexts are parsed, if the distributable element is in place in web.xml
Tomcat asks the Cluster class (in this case SimpleTcpCluster) to create a manager
for the replicated context. So with clustering enabled, distributable set in web.xml
Tomcat will create a DeltaManager for that context instead of a StandardManager.
The cluster class will start up a membership service (multicast) and a replication service (tcp unicast).
More on the architecture further down in this document.

TomcatB starts up

When TomcatB starts up, it follows the same sequence as TomcatA did with one exception.
The cluster is started and will establish a membership (TomcatA,TomcatB).
TomcatB will now request the session state from a server that already exists in the cluster,
in this case TomcatA. TomcatA responds to the request, and before TomcatB starts listening
for HTTP requests, the state has been transferred from TomcatA to TomcatB.
In case TomcatA doesn't respond, TomcatB will time out after 60 seconds, and issue a log
entry. The session state gets transferred for each web application that has distributable in
its web.xml. Note: To use session replication efficiently, all your tomcat instances should be
configured the same.

TomcatA receives a request, a session S1 is created.

The request coming in to TomcatA is treated exactly the same way as without session replication.
The action happens when the request is completed, the ReplicationValve will intercept
the request before the response is returned to the user.
At this point it finds that the session has been modified, and it uses TCP to replicate the
session to TomcatB. Once the serialized data has been handed off to the operating systems TCP logic,
the request returns to the user, back through the valve pipeline.
For each request the entire session is replicated, this allows code that modifies attributes
in the session without calling setAttribute or removeAttribute to be replicated.
a useDirtyFlag configuration parameter can be used to optimize the number of times
a session is replicated.

TomcatA crashes

When TomcatA crashes, TomcatB receives a notification that TomcatA has dropped out
of the cluster. TomcatB removes TomcatA from its membership list, and TomcatA will no longer
be notified of any changes that occurs in TomcatB.
The load balancer will redirect the requests from TomcatA to TomcatB and all the sessions
are current.

TomcatB receives a request for session S1

Nothing exciting, TomcatB will process the request as any other request.

TomcatA starts up

Upon start up, before TomcatA starts taking new request and making itself
available to it will follow the start up sequence described above 1) 2).
It will join the cluster, contact TomcatB for the current state of all the sessions.
And once it receives the session state, it finishes loading and opens its HTTP/mod_jk ports.
So no requests will make it to TomcatA until it has received the session state from TomcatB.

TomcatA receives a request, invalidate is called on the session (S1)

The invalidate call is intercepted, and the session is queued with invalidated sessions.
When the request is complete, instead of sending out the session that has changed, it sends out
an "expire" message to TomcatB and TomcatB will invalidate the session as well.

TomcatB receives a request, for a new session (S2)

Same scenario as in step 3)

TomcatA The session S2 expires due to inactivity.

The invalidate call is intercepted the same was as when a session is invalidated by the user,
and the session is queued with invalidated sessions.
At this point, the invalidated session will not be replicated across until
another request comes through the system and checks the invalid queue.

Phuuuhh! :)

Membership
Clustering membership is established using very simple multicast pings.
Each Tomcat instance will periodically send out a multicast ping,
in the ping message the instance will broad cast its IP and TCP listen port
for replication.
If an instance has not received such a ping within a given timeframe, the
member is considered dead. Very simple, and very effective!
Of course, you need to enable multicasting on your system.

TCP Replication
Once a multicast ping has been received, the member is added to the cluster
Upon the next replication request, the sending instance will use the host and
port info and establish a TCP socket. Using this socket it sends over the serialized data.
The reason I choose TCP sockets is because it has built in flow control and guaranteed delivery.
So I know, when I send some data, it will make it there :)

Distributed locking and pages using frames
Tomcat does not keep session instances in sync across the cluster.
The implementation of such logic would be to much overhead and cause all
kinds of problems. If your client accesses the same session
simultanously using multiple requests, then the last request
will override the other sessions in the cluster.

Monitoring your Cluster with JMX

Monitoring is a very important question when you use a cluster. Some of the cluster objects are JMX MBeans